Magnesium alloy sheet having improved formability at room temperature, and method for manufacturing same

ABSTRACT

Provided are a magnesium alloy sheet having improved formability at room temperature and a method for manufacturing same. According to one embodiment of the present invention, the method for manufacturing the magnesium alloy sheets having improved formability at room temperature is characterized by comprising a first pretreatment step of applying residual compression stress to the surface the magnesium alloy raw material.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to and the benefit of Korean Patent Application No. 10-2010-0105012 filed in the Korean Intellectual Property Office on Oct. 27, 2010, the entire contents of which are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a magnesium alloy sheet and a method of manufacturing the same, and more particularly, to a magnesium alloy sheet with improved normal temperature formability and a method of manufacturing the same.

BACKGROUND OF THE INVENTION

A magnesium material has a density which is just two thirds that of aluminum and one fourth that of iron, and has a thermal conductivity 1.2 times that of zinc and two times that of iron when alloyed with other metals, and has an excellent heat dissipation characteristic even when compared to plastics and the like. Further, the magnesium material has excellent electromagnetic-wave shielding property, vibration absorption property, corrosion resistance, and the like, and thus has been highlighted as a material for automobile parts and mobile electronics products, which are required to have light weight.

However, the magnesium material (pure magnesium or magnesium alloy) is different from the existing steel plate or aluminum alloy sheet in the manufacture and processing thereof because magnesium has a low elongation at a normal temperature. Magnesium has a hexagonal close packed (HCP) structure and fails to have a slip system enough to be freely modified at a normal temperature, and thus has very strong brittleness and insufficient formability which have been pointed out as limitations.

Due to the normal temperature forming limitation of the magnesium material, the magnesium material has been formed usually using a casting method such as die casting and the like in the related art. However, the casting method requires a mold to be separately manufactured, and thus is disadvantageous in that the manufacturing time and the manufacturing cost increase, the product quality is not uniform due to a high defective proportion, and it is difficult to manufacture the mold in the form of thin plate.

Meanwhile, in order to manufacture the magnesium material into a complex shape, there is a method of forming the magnesium material through press processing and the like. In the forming method using the press processing, when the magnesium material is molded at a normal temperature, destruction easily occurs, and thus a warm forming, in which forming is performed at a material and mold temperature in a range from 200° C. to 300° C., has been used.

However, in the case of the above-described warm forming, there are problems in that it is difficult to implement a continuous process for mass production, and it is not appropriate for commercializing the warm forming method due to problems such as costs, and the like.

Therefore, need for a magnesium material with improved normal temperature formability that may be formed and processed even at a normal temperature through the press processing, and the like has been increased.

SUMMARY OF THE INVENTION

Exemplary embodiments of the present invention have been made in an effort to provide a magnesium alloy sheet with improved normal temperature formability by imparting compressive residual stress to the surface of the magnesium alloy, and a method of manufacturing the same.

An exemplary embodiment of the present invention provides a method for manufacturing a magnesium alloy sheet with improved normal temperature formability, the method including a first step of performing a pretreatment which imparts compressive residual stress to a surface of a magnesium alloy raw material.

In addition, the first step may be one process of sand blast, shot peening, laser peening, or ultrasonic peening.

Furthermore, the method may further include an annealing step of annealing the magnesium alloy raw material prior to the first step.

Another exemplary embodiment of the present invention provides a magnesium alloy sheet with improved normal temperature formability, which is manufactured by using the method for manufacturing a magnesium alloy sheet with improved normal temperature formability according to an exemplary embodiment of the present invention.

Exemplary embodiments of the present invention may improve low formability of a magnesium material at a normal temperature by performing a pretreatment process for imparting compressive residual stress to the surface of a magnesium alloy raw material.

Further, a continuous process for mass production may be achieved by providing a magnesium alloy sheet with improved normal temperature formability, thereby producing a large-sized area magnesium alloy large-area part.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flowchart illustrating a method of manufacturing a magnesium alloy sheet with improved normal temperature formability according to an exemplary embodiment of the present invention.

FIG. 2A is a graph showing the degree of residual stress measured by a hole drilling method of Comparative Examples 1 to 3.

FIG. 2B is a graph showing the degree of residual stress measured by a hole drilling method of Examples 1 to 3.

FIG. 3 is a graph showing the values of tensile test results of the magnesium alloy sheets corresponding to Comparative Examples 1 to 3 and Examples 1 to 3.

FIG. 4 is a graph showing the indices of Erichsen test results of the magnesium alloy sheets pretreated by Comparative Examples 2 and 3 and Examples.

DETAILED DESCRIPTION OF THE EMBODIMENTS

FIG. 1 is a flowchart illustrating a method of manufacturing a magnesium alloy sheet with improved normal temperature formability according to an exemplary embodiment of the present invention.

Referring to FIG. 1, the method of manufacturing a magnesium alloy sheet with improved normal temperature formability includes a first step S200 of performing a pretreatment which imparts compressive residual stress to the surface of the magnesium alloy raw material.

The magnesium alloy in the present invention refers to an alloy including magnesium as a main component. Examples thereof include Mg—Al—Zn, Mg—Al, and Mg—Zn, and AZ31, AZ61, AZ80, AZ91, and AM100 which are Mg—Mn-based alloys, and the like. In addition, the magnesium alloy may also include ZK-based alloys containing Zr, which are high-strength alloys; EZ, EK, HK, HZ, and MH-based alloys containing Th and Ce, which are heat resistant alloys; AS21 and AS41 containing Al and Si, which are heat resistant cast alloys; QE22, QH21, and WE54 containing Ag, R.E, Th, Zr, and the like, which are heat resistant alloys; and LA91, L14A1 alloys and the like containing Li.

The first step S200 refers to performing a process capable of imparting compressive residual stress to the surface of the magnesium alloy in the pretreatment of the magnesium alloy raw material.

The compressive residual stress refers to a stress remaining in a material even while external forces are all removed after the material is deformed by plastic deformation.

It has been known that the process of imparting compressive residual stress is performed in order to increase the fatigue life of a metal in the related art. However, the inventors of the present invention have confirmed that when compressive residual stress is imparted to the surface of the magnesium alloy raw material in the pretreatment process of the magnesium alloy raw material, normal temperature formability of the magnesium alloy may be improved.

Therefore, the present invention is characterized in that low formability of the magnesium alloy at a normal temperature is significantly improved by adopting a process of imparting compressive residual stress as a pretreatment process of the magnesium alloy raw material.

The first step S200 may be one process of a sand blast process, a shot peening process, a laser peening process, or an ultrasonic peening process.

The shot peening is a kind of cold working by which the surface of metal is hammered by shooting a steel ball called a shot ball on the surface of metal at high speed. While the shot ball is colliding with the metal surface at high speed, the kinetic energy of the shot ball instantaneously causes plastic deformation to the surface of the material, and then the shot ball is separated from the surface.

The sand blast is a process of imparting compressive residual stress by spraying sand on the surface of the material using compressed air.

The laser peening is a process of imparting compressive residual stress through a shock wave generated by plasma pressure generated by coating the surface of the material with a layer consisting of an opaque coating film and a transparent coating film, and then irradiating a laser beam with a focal point diameter from about 2.5 mm to about 25 mm thereon.

The ultrasonic peening is a process of resonating steel balls using an ultrasonic wave, and imparting compressive residual stress by dispersing the steel balls on the surface of the metal.

Meanwhile, the process of imparting compressive residual stress is sufficient as long as the process may impart compressive residual stress to the surface of the magnesium material, and is not limited to the above-described specific examples. However, the process of imparting compressive residual stress will be described below mainly with respect to the shot peening process for convenience of the description.

The method of improving the normal temperature formability of the magnesium alloy according to an exemplary embodiment of the present invention is characterized to further include an annealing step S100 of annealing the magnesium alloy raw material prior to the first step S200.

The inventors of the present invention have additionally confirmed that in the pretreatment process of a magnesium alloy raw material, when the magnesium alloy raw material is annealed, and then compressive residual stress is imparted to the surface of the magnesium alloy raw material, the normal temperature formability of the magnesium alloy may be further improved.

The annealing refers to a heat treatment method of heating a magnesium alloy raw material at a constant temperature, and then slowly cooling the material to make the inner structure thereof even.

The heat treatment temperature in the annealing step S100 may vary depending on the conditions (type of material and pre-state), and is not limited to any specific temperature. For example, the magnesium alloy may be subjected to heat treatment at 300° C. to 400° C., and then air-cooled.

Furthermore, the heat treatment time is not limited to any specific time. For example, it is possible to work the heat treatment within 30 minutes in order to optimize the efficiency of time and cost.

As described above, exemplary embodiments of the present invention have an effect that low formability of the magnesium material at a normal temperature, which has been considered as disadvantageous in the related art, may be improved by performing a pretreatment process for imparting compressive residual stress to a magnesium alloy surface raw material.

Further, the magnesium alloy sheet with improved normal temperature formability, manufactured using the methods according to exemplary embodiments of the present invention, has an additional effect that the magnesium alloy sheet enables a continuous process for mass production to be implemented, thereby producing a large-area magnesium alloy sheet.

Hereinafter, the present invention will be described in more detail through the Comparative Examples and Examples. However, it is obvious that the following Comparative Examples and Examples are only illustrative for describing the present invention in detail, and are not intended to limit the scope of the present invention.

Example Preparation of Magnesium Alloy Raw Material

Under the conditions described in the following Table 1, raw materials corresponding to Comparative Examples 1 to 3 and Examples 1 to 4 were prepared. The base material was AZ31B, and a sheet having a thickness of 1.6 mm, which was manufactured through a Strip Casting process, was used. The Strip Casting refers to a process of manufacturing the sheet by cooling a molten metal while directly being passed between a rolling roll.

As the process of imparting compressive residual stress, a shot peening process was used. Under conditions such as a pressure of 0.8 MPa and a spray distance of 15 cm as the working conditions, a shot ball (Alumina Powder #240) was sprayed for 60 to 120 seconds.

Meanwhile, the annealing was maintained at each temperature for 20 to 60 minutes, and then air-cooling was performed.

TABLE 1 Whether annealing Whether process of is performed and imparting compressive processing residual stress is temperature performed (shot peening) Comparative Example 1 None None Comparative Example 2 Yes (345° C.) None Comparative Example 3 Yes (400° C.) None Example 1 None Yes Example 2 Yes (345° C.) Yes Example 3 Yes (400° C.) Yes Example 4 Yes (450° C.) Yes

Vickers Hardness Test

In order to confirm whether compressive residual stress is imparted to the surface of the magnesium alloy raw material, Vickers hardness tests were performed with respect to Comparative Examples 1 and 2 and Example 2. The Vickers hardness tests were performed using a Micro Vickers FM 700 hardness tester, and as the test conditions, the test load was 500 gf and the load dwell time was 5 seconds. The variations in hardness depending on the thickness direction of the magnesium alloy raw materials corresponding to Comparative Examples 1 and 2 and Example 2 were summarized in the following Table 2. Meanwhile, the hardness in Example 2 was measured by dividing the upper portion (surface), inner portion, and lower portion of the magnesium alloy raw material.

TABLE 2 Hardness when Hardness when Hardness when measurement measurement a measurement Average is made is made is made three hardness once (Hv) twice (Hv) times (Hv) (Hv) Comparative Example 1 65.9 66 65.4 65.8 Comparative Example 2 58.3 58.9 58.3 58.5 Example 2 Upper portion (surface) 98.6 84.6 88.3 90.5 Inner portion 75 75.1 70.2 73.43 Lower portion 62.9 66.5 64.9 64.76

Referring to Table 2, the hardness when the pretreatment of the raw material was not performed (Comparative Example 1) was measured as Hv65.8 (average value, hereinafter all the same). In addition, when the raw material is only subjected to the annealing treatment (Comparative Example 2), the hardness was measured as Hv58.5, and thus it can be confirmed that the raw material was softened more than the raw material which was not subjected to the pretreatment.

Meanwhile, when the annealing treatment is performed on and compressive residual stress is imparted to the raw material (Example 2), it can be confirmed that the hardness on the upper portion (surface) of the raw material was increased to Hv90.5 (maximally Hv98.6) and the hardness on the inner portion thereof was also increased to Hv73.43 (maximally Hv75.1). That is, the hardness on the surface and inner portion thereof in Example 2 was all higher than that in Comparative Example 1 or 2. Therefore, it can be seen that the surface of the magnesium sheet was hardened by plastic deformation caused by the process (shot peening) of imparting compressive residual stress thereto, and at the same time, compressive residual stress is remaining therein.

Measurement of Residual Stress Through Hole Drilling Method

In Comparative Examples and Examples, the compressive residual stress was measured by a hole drilling method according to ASTM E837-08. That is, with respect to the presence or absence of the residual stress suggested in the Vickers hardness test, quantitative results were derived.

The hole drilling method is based on a theory that when a hole is perforated on a material on which residual stress is present, the restraint around the hole is released in order to reach the equilibrium state of the stress, and the relaxed variation around the hole is measured using a strain gauge.

Devices used in the measurement were RS-200 (drilling apparatus, Vishay, USA), P-3 (Strain Indicator, Vishay, USA), and H-drill ver3.10 (Analysis S/W, Vishay, USA), and as the strain gauge, CEA-06-06UL-120 (Vishay, USA) was used.

Meanwhile, for measurement of residual stress, the center of a test specimen was defined by a second gauge and was used as a reference position, and on the positions which were 20 mm spaced apart in the left and right directions from the reference point, hole portions of first and third gauges were placed.

FIG. 2A is a graph showing the degree of residual stress of Comparative Examples 1 to 3 measured by a hole drilling method.

The (−) direction from the reference point indicates that compressive residual stress is present on the test specimen, and the (+) direction indicates that tensile residual stress is present on the test specimen. Meanwhile, N1, N2, and N3 denote the number of tests.

Referring to FIG. 2A, it can be confirmed that compressive residual stress is present when the raw material is not subjected to pretreatment (Comparative Example 1), while tensile residual stress is present when the raw material is only subjected to annealing treatment (Comparative Examples 2 and 3).

FIG. 2B is a graph showing the degree of residual stress of Examples 1 to 3 measured by a hole drilling method. S1, S2, and S3 denote the number of tests.

Referring to FIG. 2B, it can be confirmed that when compressive residual stress is imparted to the raw material (Example 1), compressive residual stress is further increased when compared to the above-described Comparative Example 1. Furthermore, it can be confirmed that when the annealing treatment was performed and then compressive residual stress was imparted (Examples 2 and 3), the tensile residual stress in the case of performing only the annealing treatment (Comparative Examples 2 and 3) was transformed into the compressive residual stress state.

Therefore, it can be seen that on the surface of the magnesium sheet, compressive residual stress was produced by a process of imparting compressive residual stress (shot peening).

Uniaxial Tensile Test

In Comparative Example 1 and Examples 1 to 3, a uniaxial tensile test was performed. In the tensile test, a test specimen (gauge length 25 mm) in accordance with the ASTM-E8M standard was used in the INSTRON-4206 equipment, and the test speed was 75 mm/min.

FIG. 3 is a graph showing the values of tensile test results of the magnesium alloy sheets corresponding to Comparative Examples 1 to 3 and Examples 1 to 3. Referring to Table 3, the graph is a stress-strain curve, and a curve showing the degree of deformation of the specimen with respect to stress applied on the specimen.

The elongation refers to a value obtained by dividing the elongated length of the specimen in the stress-strain curve by the initial length of the specimen, and is a general measure indicating the formability of the material.

First, in Comparative Example 1 in which the raw material was not subjected to pretreatment, tensile strength was measured as 285.69 Mpa, and the elongation was measured as 15%.

As shown in the graph in comparison with this, in the case of Example 1 in which only a process of imparting compressive residual stress to the raw material was performed, the tensile strength was 276.58 Mpa and the elongation was 17.62%, indicating that the values were a little improved. Further, in the case of Example 2 in which a process of annealing the raw material at 345° C. and imparting compressive residual stress thereto was performed, the tensile strength was 262.46 MPa and the elongation was 23.16%, confirming that the values were further improved.

Meanwhile, the magnesium sheet is frequently in a stress state of two or more axes, and thus it is difficult to properly judge the formability only by the uniaxial tensile test. Therefore, the formability of the magnesium sheet needs to be judged along with the Erichsen test and deep drawing test results to be described below with the uniaxial tensile test.

Formability Test (Erichsen Test)

In Examples 1 to 3 and Examples 1 to 4, formability tests were performed. As a test method of formability, an Erichsen test was used. The Erichsen test is a representative test method that judges the formability of a sheet, and refers to the measuring of the formability of the sheet by raising a ball-shaped punch upward in the compressed state, and measuring a height until the material is broken while compressing a material between upper and lower dies.

The Erichsen test was performed with the Erichsen-142/40 equipment, and the test conditions were as follows: a pressure of 0.8 Mpa, a punch speed of 0.5 mm/s, and a blank holding force of 3.5 kN.

The index of Erichsen derived from the Erichsen test, which is an index indicating the formability of the material which id deformed without being fractured, is a representative index indicating the formability in the stress state of two or more axes unlike the uniaxial tensile test. It may be interpreted that the higher the index of Erichsen is, the better the formability is.

The following Table 3 shows the indices of Erichsen of Comparative Examples 1 to 3 and Examples 1 to 4 as the above-described formability test results. In relation to this, FIG. 4 is a graph showing the indices of Erichsen test results of the magnesium alloy sheets pretreated by Comparative Examples 2 and 3 and Examples.

In the graph, the Y axis denotes the force, and the X axis denotes the stroke of the magnesium alloy sheet. Therefore, it is possible to compare formalities of magnesium alloy sheets between the above-described Comparative Examples and Examples by comparing the strokes of the magnesium alloy sheets according to the same force.

TABLE 3 Comparative Comparative Comparative Example 1 Example 2 Example 3 Example 1 Example 2 Example 3 Example 4 Index of 2.3 2.5 2.8 2.8 5.8 5.8 2.3 Erichsen

Referring to Table 3 and FIG. 4, it can be seen that comparing the case where a raw material is subjected to annealing treatment (Comparative Examples 2 and 3) or a process of imparting compressive residual stress to the raw material is performed (Example 1) with the case where the raw material was not subjected to pretreatment (Comparative Example 1), excellent results were obtained from the formability test in Comparative Examples 2 and 3 and Example 1, and thus there was an effect that the formability of the magnesium alloy sheet was enhanced.

In addition, it can be confirmed that there was an effect that the formability of the magnesium alloy sheet was further enhanced when the process of imparting compressive residual stress was sequentially performed after the annealing treatment (Examples 2 and 3) than when only the annealing treatment or the process of imparting compressive residual stress was performed.

However, even when the process of imparting compressive residual stress was sequentially performed after the annealing treatment, it can be confirmed that the formability was rather decreased when the temperature of the annealing treatment was 400° C. or more.

Deep Drawing Square Cup Forming

In Comparative Examples 1 and 2 and Example 2, deep drawing square cup formings were performed. The deep drawing processing is a representative forming method of making a seamless hollow vessel from a flat plate, and a technique in which a sidewall is made by moving a material on the surface of the die between the punch and the die while reducing the material in a circumferential direction.

In Comparative Examples 1 and 2, the forming was performed under the deep drawing square cup forming conditions as follows: a mold temperature of 100° C., a punch speed of 0.43 mm/s, a molding height of 7 mm, and a size of 40*60 mm. Meanwhile, in Example 2, except that the mold temperature was a normal temperature, the other conditions are the same as described above.

As a result of the experiment, it was observed that when the raw material was not subjected to pretreatment (Comparative Example 1) or was only annealed (Comparative Example 2), fracture occurred, and thus, the square cup forming failed to be achieved.

However, when the raw material was subjected to annealing and a process of imparting compressive residual stress as a pretreatment (Example 2), it can be confirmed that there was no fracture even though the forming is performed at a normal temperature, and the square cup forming could be achieved, thereby further improving formability.

As described above, exemplary embodiments of the present invention have been described, but it should be understood that those skilled in the art may modify and change the present invention in various ways without departing from the spirit of the present invention described in the claims by the addition, change, deletion or addition of constituent elements, and that the modifications and changes are included in the claims of the present invention. 

1. A method of manufacturing a magnesium alloy sheet with improved normal temperature formability, the method comprising: a first step of performing a pretreatment which imparts compressive residual stress to a surface of a magnesium alloy raw material.
 2. The method of claim 1, wherein the first step is one process of sand blast, shot peening, laser peening, or ultrasonic peening.
 3. The method of claim 1, further comprising: an annealing step of annealing the magnesium alloy raw material prior to the first step.
 4. A magnesium alloy sheet with improved normal temperature formability manufactured by the method of claim
 1. 